Vascular biology of thrombosis: platelet-vessel wall interactions and aspirin effects.

The antithrombotic effect of aspirin has long been recognized, and administration of low doses (80-160 mg/day) for the prevention of ischemic events in patients with coronary artery disease (CAD) is now generally considered to be routine practice. The action of aspirin derives mostly from the selective inhibition of cyclo-oxygenases (Cox). These enzymes (Cox-1 and Cox-2) catalyze the synthesis of eicosanoids, which play an important part in platelet-vessel wall interactions. Cox-1 catalyzes the synthesis of thromboxane A2 (Tx-A2), which causes platelet activation, vasoconstriction, and smooth muscle proliferation. Tx-A2 levels are elevated in conditions associated with platelet activation, including unstable angina and cerebral ischemia. Conversely, Cox-2 controls the synthesis of prostacyclin (PGI2), a local platelet regulator with an effect opposite to that of Tx-A2. PGI2 is produced as a compensatory response to increases in Tx-A2 during ischemic events. Aspirin is a more potent inhibitor of Cox-1 than of Cox-2, unlike other non-steroidal anti-inflammatory drugs (NSAIDs), which have limited selectivity. Aspirin at low doses selectively inhibits the formation of Tx-A2 without inhibiting the basal biosynthesis of cardioprotective PGI2. Furthermore, aspirin causes complete enzyme inhibition, without the recovery of enzyme activity at trough drug levels associated with conventional NSAIDs. The effect of aspirin in the prevention of ischemic events has been well documented in many recent clinical trials involving more than 50,000 patients with CAD. It is clear from these studies that aspirin, alone or in combination with other antithrombotics, significantly reduces the incidence of cardiovascular death, stroke, and myocardial infarction.